The present invention relates to an illumination device, and more particularly to a laser illumination device based on electrically switchable Bragg gratings that reduces laser speckle.
Miniature solid-state lasers are currently being considered for a range of display applications. The competitive advantage of lasers in display applications results from increased lifetime, lower cost, higher brightness and improved colour gamut. Laser displays suffer from speckle, a sparkly or granular structure seen in uniformly illuminated rough surfaces. Speckle arises from the high spatial and temporal coherence of lasers. Speckle reduces image sharpness and is distracting to the viewer.
Several approaches for reducing speckle contrast have been proposed based on spatial and temporal decorrelation of speckle patterns. More precisely, speckle reduction is based on averaging multiple sets of speckle patterns from a speckle surface resolution cell with the averaging taking place over the human eye integration time. Speckle may be characterized by the parameter speckle contrast which is defined as the ratio of the standard deviation of the speckle intensity to the mean speckle intensity. Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast. Traditionally, the simplest way to reduce speckle has been to use a rotating diffuser to direct incident light into randomly distributed ray directions. The effect is to produce a multiplicity of speckle patterns while maintaining a uniform a time-averaged intensity profile. This type of approach is often referred to as angle diversity. Another approach known as polarization diversity relies on averaging phase shifted speckle patterns. In practice neither approach succeeds in eliminating speckle entirely.
It is known that speckle may be reduced by using an electro optic device to generate variations in the refractive index profile of material such that the phase fronts of light incident on the device are modulated in phase and or amplitude. The published International Patent Application No. WO/2007/015141 entitled LASER ILLUMINATOR discloses a despeckler based on a new type of electro optical device known as an electrically Switchable Bragg Grating (SBG). An (SBG) is formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture. Typically, SBG devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. Techniques for making and filling glass cells are well known in the liquid crystal display industry. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. A volume phase grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied. U.S. Pat. No. 5,942,157 and U.S. Pat. No. 5,751,452 describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. An SBG device typically comprises at least one SBG element that has a diffracting state and a non-diffracting state. Typically, the SBG element is configured with its cell walls perpendicular to an optical axis. An SBG element diffracts incident off-axis light in a direction substantially parallel to the optical axis when in said active state. However, each SBG element is substantially transparent to said light when in said inactive state. An SBG element can be designed to diffract at least one wavelength of red, green or blue light. SBGs may be stacked to provide independently switchable layers.
SBGs with Bragg grating pitches much smaller than the operating wavelength exhibit form birefringence in other words they behave like a negative uniaxial crystal with an optic axis perpendicular to the Bragg planes. They are referred to as sub-wavelength gratings. The incident wave cannot resolve the sub-wavelength structures and sees only the spatial average of its material properties. Only zero order forward and backward “diffracted” waves propagate and all higher diffracted orders are evanescent. The birefringence is switched off when the refractive indices of the PDLC and polymer planes are equal. The retardance of a sub wavelength grating is defined as the difference between the extraordinary and ordinary refractive indices multiplied by the grating thickness. As will be discussed later subwavelength gratings can be used to provide a variable refractive index medium.
There are two types of speckle known as objective speckle and subjective speckle. Objective speckle occurs as a two dimensional random pattern on a projection screen and has the effect of degrading the resolution of the projected image. Subjective speckle manifests itself as floating light spots that the eye cannot focus on. It does not affect the image on the screen surface. Classical methods for overcoming speckle rely on the principle of randomly displacing a diffusing surface relative to the laser illumination beam. The relative displacement is usually provided by a rotating diffusing screen. Another equivalent solution is to have a static diffusing screen and a means for scanning the laser illumination across the screen. However, such approaches have failed to deliver the levels of speckle contrast reduction required by modern laser display technology. Mechanical scanning solutions also suffer from the problems of mechanical and optical design complexity, noise and cost of implementation. There is a need for a compact solid state solution to the problem of speckle reduction using the principle of angular diversity.
There is a requirement for a despeckler with improved speckle contrast reduction.